As an exhaust gas purification system for purifying NOx in an exhaust gas from a diesel engine, an SCR system using an SCR (selective catalytic reduction) device has been developed.
The SCR system supplies urea solution to an exhaust gas upstream of the SCR device to generate ammonia using heat of the exhaust gas, and using the ammonia, reduces NOx on an SCR catalyst to purify NOx (see, for example, patent literature 1).
As illustrated in FIG. 4, upon urea solution reaching a temperature equal to or exceeding its boiling point, water in the urea solution starts to evaporate, and as a result, the urea solution turns into a melt, through which a gas is generated. The generated gas, upon its temperature being raised, is crystallized, resulting in generation of a solid. Upon the generated solid subliming, ammonia is generated. Also, the generated melt and/or gas are partially hydrolyzed with water vapor, generating ammonia and carbon dioxide.
Urea solution is injected from a dosing valve (urea injection device or dosing module) provided on the upstream side of the SCR device.
As illustrated in FIG. 5(a), a dosing valve 104 has a structure in which a cylinder 51 filled with high-pressure urea solution L is provided with an injection orifice 52 and a valving element 53 occluding the injection orifice 52 is attached to a plunger 54, and upon energization of a coil (not illustrated) to pull up the plunger 54, the valving element 53 is moved away from the injection orifice 52, whereby urea solution L is injected. Upon the energization of the coil being stopped, the plunger 54 is pulled down by means of an internal spring force so that the valving element 53 occludes the injection orifice 52, whereby the injection of urea solution L is stopped.
The dosing valve 104 is provided in an exhaust pipe for an engine, the exhaust pipe allowing a high-temperature exhaust gas to flow therein. Upon the dosing valve 104 heated to a high temperature, the urea solution L in the dosing valve 104 also has a high temperature, and disadvantageously, proper urea solution injection control may not be performed as a result of the dosing valve 104 having a failure in operation, e.g., the urea solution L in the dosing valve 104 being crystallized, resulting in fixation of the plunger as illustrated in FIG. 5(b) or a solid body resulting from the crystallization of the urea solution L being stuck between the valving element 53 and the cylinder 51 (valve seat) as illustrated in FIG. 5(c), disabling injection of urea solution L to be stopped.
Furthermore, a corrosive intermediate product (ammonium carbamate) is disadvantageously generated in a process of generating ammonia from urea solution and the corrosive intermediate product corrodes the inside of the dosing valve. Furthermore, when urea solution in the dosing valve is heated to a high temperature, parts made of rubber or resins, which are used in the dosing valve, may disadvantageously be damaged. In the present description, these troubles are collectively referred to as thermal damage.
In order to prevent such thermal damage, cooling water for cooling an engine is made to pass through a dosing valve to cool the dosing valve by means of the cooling water.